Research and analysis

Impacts on groundwater quality from abandoned hydrocarbon wells - summary

Published 22 October 2019

1. Chief Scientist’s Group research summary

Evidence from the USA suggests that hydrocarbon wells, including shale gas wells, have had an impact on some shallow groundwater. There is a general consensus that a failure of well integrity, or poorly constructed wells, is the main cause of subsurface leakages.

Over 2,000 onshore deep wells have been drilled in the UK for hydrocarbon exploration or production since 1900. Approximately two thirds of these have since been decommissioned in line with regulations and guidance in place at the time, leaving them in a safe state, often described as ‘abandoned’ by the industry.. Although there are currently no reports of groundwater pollution from these wells, there is some evidence of limited methane emissions to air from an earlier study of abandoned wells carried out by the Environment Agency and experts from the University of Durham (project SC140032).

The British Geological Survey and the Environment Agency identified where abandoned wells had boreholes nearby that could be used to monitor groundwater quality. Two former oil wells and 2 former gas wells were selected for field monitoring. No pollution was found that might be related to leakages from the oil and gas wells.

1.1 Why was this project needed?

Systematic monitoring of groundwater has not been carried out around abandoned wells and very little data exist. A better understanding is needed of the long-term integrity of abandoned deep wells to identify:

• if leakage has occurred
• what impact there has been on groundwater quality

1.2 What did the project involve?

Records for 2,000 oil and gas wells were reviewed to identify which were most likely to have given rise to impacts on groundwater. The factors considered included:

• whether the well had been drilled for oil or gas
• the depth at which hydrocarbons were found
• how long oil or gas had been produced from the well
• the length of time since the well was abandoned
• the nature of the overlying aquifers

An important consideration was also whether there were wells or boreholes nearby that could be used to cost-effectively monitor groundwater. Twenty seven oil and gas wells were identified as suitable for further consideration.

From this review, it was possible to select the areas around 2 former gas wells (Nooks Farm in Staffordshire and Ashdown in Sussex) and 2 former oil wells (Hemswell in Lincolnshire and Lomer in Hampshire) for field investigation. However, many of the sampling points identified in the desk study were found to be unsuitable for sampling as they were disused, decommissioned, unsafe or lacked access. Those sites found to be suitable were sampled twice during the project, with a total of 48 groundwater samples from shallow aquifers being collected

1.3 What were the main findings?

In both sampling rounds, only low concentrations of hydrocarbons were found in groundwater. In the first sampling round, a high maximum dissolved methane concentration of 407μg per litre was recorded. However, this value was not repeated in the second sampling round. Although relatively high, this concentration is not enough to allow further tests to be performed to determine whether the gas is from the decay of organic material or from a hydrocarbon reservoir.

Organic compounds were detected in some samples at low concentrations. The substances included volatile organic compounds, polyaromatic hydrocarbons, pesticides, herbicides, fungicides, surfactants, analgesics and veterinary compounds. None of the detections could be linked unequivocally to the presence of abandoned hydrocarbon wells and many were clearly due to other anthropogenic activities.

In view of the difficulty in finding suitable sampling sites, a soil gas survey was carried out in 2 areas: Ashdown, one of the original study areas; and a new location at Bolney (also in Sussex).

Due to poor ground conditions at the time of sampling, the results were ambiguous, though showing elevated concentrations of both carbon dioxide and methane. Further work in dry ground conditions would be required to say with certainty that these concentrations were linked directly to the presence of the gas wells.

1.4 What is the next step?

This study did not identify at the sites selected the impacts on groundwater that have been reported in similar, though more intensive, studies in the USA. Unlike the US studies, however, this study used existing groundwater boreholes and no dedicated monitoring boreholes were installed near the study sites.

The findings indicate that, if any contamination exists at the sites studied, then this is not widespread. It would be valuable to undertake monitoring closer to the oil and gas wells, but this would involve a costly drilling and monitoring programme. This will be considered as a possible future research need.

1.5 Publishing details

This summary relates to information from project SC150024 reported in detail in the following output:

Report:

SC150024

Title:

Impacts on groundwater quality from abandoned hydrocarbon wells

October 2019

Project Manager:

Ian Davey, Research, Analysis and Evaluation

Research Contractor:

British Geological Survey, Keyworth, Nottingham NG12 5GG

This project was funded by the Environment Agency’s Research, Analysis and Evaluation group, which provides scientific knowledge, tools and techniques to enable us to protect people and the environment.

Enquiries: research@environment-agency.gov.uk

© Environment Agency